The invention relates to an optical imaging device, in particular a lens system, with at least one system diaphragm, an aperture of the system diaphragm being adjustable in its opening diameter.
The use of diaphragms of various types as system diaphragms in optical imaging devices is generally known. These diaphragms, which can be changed in their opening diameter, allow the diameter of the bundle of rays passing through the optical imaging device to be continuously varied. As a result, depending on the application, the optical imaging quality can be influenced with regard to, for example, the resolution, contrast or depth of field.
Especially popular are so-called iris diaphragms, which have at least fourxe2x80x94but usually morexe2x80x94thin leaves, which are generally sickle-shaped and are rotatably mounted at one end in a fixed mount. The other end is in this case provided with a pin as a guiding device, which is inserted in a groove or slotted guideway of a rotatable ring such that, by turning the rotatable ring, the leaves can be moved in such a way that the remaining opening diameter of the diaphragm can be varied.
In the case of high-performance lens systems, in particular for use in the exposure of lithographically produced semiconductor devices, increasingly complex techniques are being used for optimizing the quality of the imaging device. Optimizing the elements which exclusively act optically, such as surface coatings of the lenses for example, has in this respect largely been taken to the technical limits, so that further increases in imaging quality can be realized only with very great expenditure.
It is therefore the object of the invention to improve the optical properties of the imaging device, such as the telecentering error or other image errors for example, by optimizing the optical-mechanical devices, in particular the system diaphragms in an imaging device.
This object is achieved according to the invention by the position of the aperture of the system diaphragm being fixed in dependence on the opening diameter of the system diaphragm.
Very complex mathematical or optical calculations from the field of optics have shown in a surprising way that changing the position of the system diaphragm in dependence on its opening diameter makes it possible to obtain an improvement in quality with respect to the imaging quality of the optical imaging device. In this respect, changing the axial position of the system diaphragm is undoubtedly the preferred movement of the latter, but a sideward or tilting movement or any desired combination of the three movements mentioned above are also conceivable.
Given that the ways of optimizing the lens systems have largely been exhausted, the benefits and advantageous improvements which can be achieved by the system diaphragm according to the invention bring about a comparatively simple and favorable possible way of enhancing imaging quality.
In particular in the area of the production of semiconductor devices or computer chips, especially high requirements have to be imposed on the imaging quality and resolution of the imaging devices. This is because the imaging devices are used for imaging extremely small patterns, just still able to be produced mechanically, for a chip layout in a reduced form on a silicon wafer coated with a light-sensitive layer. Using various etching methods, which the silicon wafer passes through, the circuits of the computer chip or semiconductor device are then formed from the microstructures exposed in this way. Extremely small deviations and distortions caused by the imaging device in the imaging of the pattern for the chip layout on the silicon wafer are themselves sufficient to lead to line contacts, short-circuits or other electronic malfunctions in the microstructures of the finished computer chip. Therefore, specifically in the case of the imaging devices used in the production of computer chips and semiconductor devices, even extremely small improvements in quality bring great advantages, for which reason the optical benefits of the system diaphragm which is adjustable in its axial position are shown to their full advantage here.
In a particularly favorable embodiment of the invention, changing the axial position of the system diaphragm in dependence on its opening diameter is achieved in a construction of a comparatively simple design. The system diaphragm has in this case at least two diaphragms arranged at an axial distance from one another, what is respectively another diaphragm being optically active in dependence on the opening diameter of the system diaphragm. Optically active means in this case that the optically active diaphragm or its optically effective edge provides the lateral limitation of the bundle of rays in the imaging device.
This double or multiple diaphragm according to the invention has the advantage that the effect according to the invention can be achieved here without complex mechanisms for adjusting the axial position. In this case, the at least two diaphragms are arranged such that they lie in at least two different planes which are axial with respect to the optical axis of the system diaphragm. As a result of the fact that in each case only one of the diaphragms is optically active, a different plane and consequently a different axial position of the system diaphragm can be realized according to the opening diameter, whereby a good optical effect is already achieved. Critical in this case is the area when two of the diaphragms xe2x80x9coverridexe2x80x9d, that is when, at a certain opening diameter of the system diaphragm, the optically effective edges of two of the diaphragms are briefly optically active at the same time. To avoid blurring effects, this requires a very precise mechanism, such as for example the mounting of the diaphragms or their individual parts by means of rolling bearing elements prestressed without any backlash.
In a further very favorable embodiment of the invention, the system diaphragm has leaves which are arranged between two diaphragm bases which are rotationally movable relatively with respect to one another, at least one of the diaphragm bases being rotationally movable and the unit comprising the leaves and the two diaphragm bases being movable in the axial direction.
This variant of a solution for achieving the object in a way according to the invention manages with only a single diaphragm. In addition to the at least two discrete axial planes of the embodiment described above, this offers the advantage that all the axial intermediate planes are available. This is because, depending on the opening diameter of the diaphragm, the latter can change its axial position, since the unit comprising leaves and diaphragm bases which are movable relatively with respect to one another is coupled by mechanical guiding elements in such a way that the desired adjustment of the axial position of the diaphragm can be brought about in dependence on the opening diameter.
This structural design has the particular advantage that a guiding groove which coordinates the rotational and axial movements of the two diaphragm bases with respect to one another can be made in any desired geometrical form. Depending on the form of the guiding groove, different mathematical dependences between the rotational movement, and consequently the changing of the opening diameter, and the axial movement of the system diaphragm can in this case be realized.
In a further very favorable variant of the invention, the diaphragm is likewise formed by means of leaves, the leaves and the surfaces of the diaphragm bases facing the leaves being arranged over the at least approximately greatest part of the opening diameter of the system diaphragm at an angle with respect to the optical axis of the system diaphragm.
As a result, the optically effective edge of the diaphragm can, depending on the geometrical shape of the leaves and of the surfaces of the diaphragm bases facing the leaves, be moved on, for example, a lateral surface of a cone or on the lateral surface of a spherical cap. The leaves, arranged in a rotationally symmetrical manner with respect to the optical axis of the system diaphragm, then move into the light path of the imaging device, for example when the system diaphragm is closed, in a linear or semicircular dependence between the opening diameter and the axial position.